The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Heat extraction machines of the initially named kind are generally known. The refrigerant is evaporated and overheated in the evaporator, i.e. is heated above its saturation temperature. Overheating the refrigerant therefore means an increase in the refrigerant temperature beyond its saturation temperature at a constant pressure. The overheating is defined as the difference between the actual temperature of the refrigerant, e.g. in the region of the evaporator outlet, and the evaporation temperature or saturation temperature of the refrigerant.
Usually, in a conventional heat extraction machine, a predetermined value is preset for the overheating of the refrigerant and the overheating is regulated such that it does not substantially differ from the preset value—independently of other operating conditions—in order to achieve an optimum efficiency of the heat extraction machine, on the one hand, and to ensure a complete evaporation of the refrigerant, on the other hand. A typical value for the overheating amounts, for example, to 6 K up to 10 K.
In known heat pumps, it has proved to be a problem that the temperature of the evaporated refrigerant reaches such high values at the outlet of the compressor under specific operating conditions, for example at particularly low external temperatures, that a degradation of oil, e.g. lubricating oil, located in the compressor takes place and/or mechanical wear of the compressor is caused. This can result in damage to the compressor and require the switching off of the heat pump or of the compressor. In addition, at particularly low external temperatures, there is the risk of the evaporator icing up, which can likewise make it necessary to switch off the heat pump or to switch over a switch valve, in particular a four-way switch valve, arranged between the compressor and the condenser or evaporator for this purpose, for the deicing of the evaporator.
Both the switching off of the heat pump for the avoidance of an increased end compression temperature at the outlet of the compressor and the switching off of the heat pump or switching over of the switch valve for the deicing of the evaporator signify standstill times of the heat pump which impair the economy of the heat pump.